Synchronous power converters are widely used in computer and telecommunication electronics. Such power converters typically convert 48 volt bus power to lower voltages (e.g. 12V, 5V and 1.2V) needed for operating microprocessors and the like.
Modern electronics are decreasing in size and typically require increased operating current at lower voltages. As a result, power converters must provide more power while occupying less circuit board space. In order to meet these requirements, power converters must operate at higher frequencies. A high operating frequency allows for smaller passive components. However, high operating frequencies greatly increase body diode conduction losses, reverse recovery losses and switching losses. These losses must be reduced in order to increase the operating frequency of next-generation power converters.
Another problem with high frequency operation is switching noise. The reverse-recovery charge of the switches creates high voltage spikes. When high voltage spikes are present, switches tolerant of high voltages must be used. High voltage-rating switches typically have high ON-state resistance (Rds(on)) and therefore higher conduction loss. Hence, a reduction of the voltage spikes would allow the use of low voltage, low loss switches and would provide an increase in operating efficiency.
FIG. 1 shows a conventional power converter with a full bridge primary circuit 10 and a center-tapped secondary circuit 12. The secondary circuit has an output capacitor Co and an output inductor Lo. The output capacitor is as large as possible, with typical values of about 300 microfarads. A load (not shown) is connected across the output capacitor Co. FIG. 2 shows a timing diagram illustrating the typical operation of the circuit of FIG. 1. Horizontal lines indicate the ON times of switches Q1 Q2 Q3 Q4 S1 and S2. As illustrated, these switches are operated in two groups, Q2, Q3 and S2 and Q1, Q4 and S1, and the groups are operated in an alternating, generally complementary fashion. However, some so-called dead time 14 is provided between the on time of the respective groups to prevent shorting of Vin or the secondary winding of the transformer. During dead times 14, the body diodes of secondary switches S1 S2 conduct current pulses 16 due to the inductance of the transformer and/or any output inductance which may be included in the power converter circuit, resulting in substantial energy loss and reverse recovery loss. Also, high voltage spikes 18 and ringing occur across the switches S1 S2. The body diode conduction loss and reverse recovery loss increase as operating frequency increases, and result in lower power conversion efficiency at higher switching frequencies.
It would be an advance in the art of power conversion to provide a power converter with reduced body diode conduction loss, reduced reverse recovery loss and reduced switching noise. Such circuits could operate at high frequency and have very small size because the higher frequency switching allows passive components of reduced size to be employed.